Electroluminescent devices typically consist of an electroluminescent layer, sometimes sandwiched between dielectrics, with electrodes attached to both sides. With the application of a voltage on the electrodes, light is emitted from the electroluminescent layer and through the electrodes, if they are transparent. When viewed under high ambient light illumination, the light that reaches the observer is composed of the emitted light and the ambient light reflected by the device. If L.sub.on is the emitted luminance, R is the reflectance by the device and L.sub.amb is the ambient illuminance, then the signal-to-reflected ambient light ratio (SRA) can be written as EQU SRA=L.sub.on /(R.multidot.I.sub.amb). (1)
Clearly, a high SRA is desirable and under a given ambient light illumination, this can be achieved by either increasing the brightness or decreasing the reflectance of the device.
Of particular interest are the devices capable of displaying images. Electronic displays, particularly light emitting displays, are frequently limited in applications involving high ambient light levels, e.g. direct sunlight, because of their high reflectance of ambient light and, hence, the tendency of the displayed imagery to "wash-out".
An image on a display is formed using activated picture elements (ON pixels with a luminance L.sub.on) of the display screen which have a higher luminance relative to the screen background. The screen background for a particular image is comprised of inactive pixels (OFF pixels with a luminance L.sub.off) along with the passive elements of the display, i.e. the area between the pixels.
The legibility of a displayed image can be quantitatively defined in terms of a contrast ratio. If the reflectance of the pixels is R and the ambient illuminance is I.sub.amb then, if the passive elements of the display are ignored for simplicity, the contrast ratio (CR) can be written as, EQU CR=(L.sub.on +R.multidot.I.sub.amb)/(L.sub.off +R.multidot.I.sub.amb). (2)
If the contrast ratio is less than a certain value, degraded legibility results and the display may be judged inadequate. For high ambient illumination, the contrast ratio deteriorates and CR tends to unity unless either L.sub.on is increased or else the reflectance of the ambient light by the pixels can be minimized.
Previous attempts to improve the contrast ratio, and hence the legibility of electronic displays, have included the use of anti-reflecting coatings on the outside of displays and auxiliary filter elements such as polarizers, band-pass filters, neutral density filters, louvred screens, plastic meshes, etc. With some of these methods, the brightness of the display has been compromised, sometimes to an unacceptable degree. Increasing the luminance of the pixels to compensate for this effect can lead to a shorter life and reliability problems.
One important electro-optical device is the electroluminescent device which has several distinguishing attributes: low power, potentially high contrast, light weight, wide viewing angle, nonlinear luminescence versus voltage characteristics important for matrix addressing, and a multi-colour capability. A typical ac electroluminescent device is composed of the electro-optical members functioning as,
a) a front transparent electrode,
b) a first transparent dielectric,
c) an electroluminescent member,
d) a second dielectric and,
In some cases, the dielectrics (b) and (d) are omitted allowing dc operation of the device. With the application of a voltage across the electrodes, a high electric field can be generated across the electroluminescent member which results in electroluminescence. The dielectric layers are used to limit the current in the device and prevent a catastrophic breakdown of the electroluminescent member, however, the device is then electrically a capacitor, hence, requiring an ac voltage to be applied across the electrodes. For large area displays, the counter electrode must have a low electrical resistivity, and hence, this usually requires it be made out of a metal such as aluminum. Unfortunately, this results in a device having a large reflectance in the visible portion of the optical spectrum.
One particular application of the electroluminescent device is a display in which pixels or more generally patterns, are formed when areas of the front (a) and rear (e) electrodes partially overlie one another in the viewing direction. The metal counter electrode reflects the ambient light strongly, and hence, under high ambient light illumination, results in a low contrast ratio for the display.
There is a need for an electroluminescent device wherein the contrast ratio of the device is significantly improved by the device having a low overall reflectance of the ambient light. The applicants have found that this can be accomplished according to the present invention, using the thin film phenomenon known in the art as optical interference.
Optical interference is defined as the variation of electromagnetic wave amplitude with distance or time, caused by the superposition of two or more waves of electromagnetic radiation (light). These two or more waves can be the result of reflection or transmission at the interfaces of thin film multilayer structures, used in the present invention, provided that the thicknesses of the individual films and layers are sufficient to support optical interference at the wavelengths of interest.
It has been proposed in U.S. Pat. No. 4,287,449, dated Sept. 1, 1981, "Light-Absorption Film for Rear Electrodes of Electroluminescent Display Panel", M. Takeda et al, to provide a display panel having at least one light absorbing layer placed between a rear dielectric layer and a counter electrode for absorbing ambient light transmitted through a transparent electrode. A plurality of light absorbing layers may be formed in the same arrangement. Materials useful for the light absorbing layers are Al.sub.2 O.sub.3, Al.sub.2 O.sub.3-x, Mo, Zr, Ti, Y, Ta, Ni, Al or the like with a thickness of about 10-300 .ANG..
Intrinsic absorption of light is defined as the process whereby some of the energy of electromagnetic radiation (light) is transferred to a substance on which it is incident or which it traverses. It is possible to significantly enhance the amount of absorption of light in a thin film multilayer structure, through the phenomenon of optical interference, over that of intrinsic absorption along provided, that as stated above, the thickness of the individual films and layers are sufficient to support optical interference at the wavelengths of interest. This phenomenon is hereinafter referred to optical interference enhanced absorption.
The Takeda et al devices, while useful, are primarily concerned with intrinsic absorption alone of the ambient light because the layers are of thickness which are not capable of minimizing reflectance of light by optical interference enhanced absorption.